1. Field of the Invention
The present invention relates to a noncontact power feed system that feeds power by using resonance phenomena such as magnetic field resonance and electric field rescnance, an apparatus used in the system, and a noncontact power feed method used for the system and the apparatus.
2. Description of the Related Art
As techniques of enabling electric energy to be transmitted in a noncontact manner, there are an electromagnetic induction system and a magnetic field resonance system. The electromagnetic induction system and the magnetic field resonance system are different from each other in various points as described below, and in recent years, an energy transmission using the magnetic field resonance system has attracted attention.
FIG. 11 is a block diagram showing a structural example of a magnetic-field-resonance-type noncontact power feed system in which a power feed source and a power feed destination have a one-to-one relationship. The magnetic-field-resonance-type noncontact power feed system shown in FIG. 11 includes a power feed source 100 and a power feed destination 200.
As shown in FIG. 11, the power feed source 100 such as a charging base includes an AC power source 101, an excitation device (coupling device) 102, and a resonance device 103. Further, the power feed destination 200 such as a cellular phone terminal includes a resonance device 201, an excitation device (coupling device) 202, and a rectifier circuit 203.
The excitation device 102 and the resonance device 103 of the power feed source 100, and the resonance device 201 and the excitation device 202 of the power feed destination 200 are each constituted of an air-cored coil. Inside the power feed source 100, the excitation device 102 and the resonance device 103 are strongly coupled to each other by electromagnetic induction. Similarly, inside the power feed destination 200, the resonance device 201 and the excitation device 202 are strongly coupled to each other by electromagnetic induction.
When a self-resonant frequency of the resonance device (air-cored coil) 103 of the power feed source 100 coincides with that of the resonance device (air-cored coil) 201 of the power feed destination 200, a magnetic field resonance relationship is obtained, in which a coupling amount becomes a maximum and a loss becomes a minimum.
Specifically, in the noncontact power feed system shown in FIG. 11, AC power (energy such as alternating current) having a predetermined frequency from the AC power source 101 is first supplied to the excitation device 102, which induces AC power in the resonance device 103 by electromagnetic induction in the power feed source 100. In this case, a frequency of the AC power that is generated in the AC power source 101 is set to be the same as the self-resonant frequencies of the resonance device 103 of the power feed source 100 and the resonance device 201 of the power feed destination 200.
As described above, the resonance device 103 of the power feed source 100 and the resonance device 201 of the power feed destination 200 are provided in the magnetic field resonance relationship. Therefore, the AC power (energy such as alternating current) is supplied from the resonance device 103 to the resonance device 201 in a noncontact manner at the self-resonant frequency.
In the power feed destination 200, the AC power supplied from the resonance device 103 of the power feed source 100 is received by the resonance device 201. The AC power from the resonance device 201 is supplied to the rectifier circuit 203 via the excitation device 202 by electromagnetic induction, and converted into DC power and output from the rectifier circuit 203.
Thus, the AC power is supplied from the power feed source 100 to the power feed destination 200 in a noncontact manner. It should be noted that the DC power output from the rectifier circuit 203 is supplied to a charging circuit connected with a battery, and used for charging the battery.
In the noncontact power feed system in which the power feed source 100 and the power feed destination 200 that are structured as shown in FIG. 11 have a one-to-one correspondence, the following features are found.
The noncontact power feed system has a relationship as shown in FIG. 12A, between a frequency of an AC power source and a coupling amount. As is found from FIG. 12A, the coupling amount is not increased even when the frequency of the AC power source is low or high, but the coupling amount becomes a maximum only at a specific frequency at which a magnetic field resonance phenomenon is caused. In other words, frequency selectivity is shown by magnetic field resonance.
Moreover, the noncontact power feed system has a relationship as shown in FIG. 12B, between a distance from the resonance device 103 to the resonance device 201, and a coupling amount. As is found from FIG. 12B, the coupling amount is smaller as the distance between the resonance devices is larger.
However, a shorter distance between the resonance devices does not increase the coupling mount, and a distance at which the coupling mount becomes a maximum exists at a certain frequency. Further, it is found from FIG. 12B that if the distance between the resonance devices has a certain range, a coupling mount above a certain level can be ensured.
In addition, the noncontact power feed system has a relationship as shown in FIG. 12C, between a resonant frequency and a distance between the resonance devices at which a maximum coupling mount is obtained. Specifically, it is found that when the resonant frequency is low, the distance between the resonance devices is large. Further, it is found that when the resonant frequency is high, a maximum coupling mount is obtained by narrowing the interval between the resonance devices.
In an electromagnetic-induction-type noncontact power feed system that has been widely used in recent years, it is necessary to share a magnetic flux between a power feed source and a power feed destination and to arrange the power feed source and the power feed destination very close to each other in order to efficiently transmit power, in which alignment of coupling axes is also important.
On the other hand, in the noncontact power feed system using the magnetic field resonance phenomenon, it is possible to transmit power with the resonance devices being more away from each other than in the electromagnetic induction system by the principle of the magnetic field resonance phenomenon, as described above. In addition, this noncontact power feed system has an advantage that transmission efficiency is not decreased so much even if the alignment of axes is not favorable.
In summary, as shown in FIG. 13, there are differences between the magnetic-field-resonance-type noncontact power feed system and the electromagnetic-induction-type noncontact power feed system. As shown in FIG. 13, the magnetic-field-resonance-type noncontact power feed system has an advantage in a deviation between transmission/reception coils (resonance devices), with the result that a transmission distance can be made longer.
Accordingly, in the case of the magnetic-field-resonance-type noncontact power feed system, it is possible to charge a plurality of power feed destinations (cellular phone terminals) by placing them on one power feed source (charging base) as shown in FIG. 14.
It should be noted that US Patent Application No. 2007/0222542 discloses a technique regarding a power transmission system using the magnetic field resonance system as described above.